1. Field of the Invention
The invention relates to a bushing that permits a brake rotor secured by bolts to thermally expand while protecting the brake rotor from deformation by the compressive impact of the bolts acting on the rotor.
2. Background Information
Friction brakes are used in cars, trains, airplanes, elevators, and other machines. In order to slow or stop an automobile, a driver steps on a brake pedal. Through mechanical linkage, the movement of the brake pedal is transmitted to a pair of fixed brake pads, between which is a brake rotor that rotates as the axle of the automobile turns from power transmitted by an engine. The brake rotor is fixed to the hub of a vehicle axle by an array of drive pin/bolt combinations radially distributed about the axle. Each drive pin/bolt combination is placed through opposite ends of apertures in the brake rotor and the hub. The brake rotor and the hub are secured to one another by tightening each bolt into its counter part drive pin.
As the driver applies force to the brake pedal, that force is transmitted as friction to the moving brake rotor by the fixed brake pads so as to slow the vehicle down or bring it to rest through controlled slippage. The rotational energy absorbed by the controlled slippage is converted into heat, principally within the brake rotor. As the brake rotor heats up, the brake rotor expands radially away from the vehicle axle. In high speed applications such as a high performance race car, the thermal expansion of the brake rotor especially is acute. If a brake rotor used in a high speed/high temperature application is not permitted to expand radially, the brake rotor would cone or warp the outer area surrounding the bolts.
To prevent coning and warping of the brake rotor area, brake designers conventionally use radially extending slots to form the drive pin/bolt apertures in the brake rotor. The length of each slot permits the brake rotor to expand and contract radially with the change in temperature of the brake rotor. The width of each radially slot works to control the rotational and axial movement of the brake rotor relative to the bolts and the brake pads.
A larger problem with the heating of a brake rotor is fading. As the temperature of the brake rotor increases, the rotor reaches a temperature where materials performance is adversely affected. As the rotor reaches this temperature, the frictional force between the brake pads and the brake rotor decreases. This phenomenon is called fading. Minimizing or preventing fading drives brake designers to focus on design techniques that dissipate brake rotor heat.
One principle technique used in the industry to dissipate brake rotor heat is the careful selection of the brake rotor material. Brake rotors are conventionally constructed from a variety of materials, including steel, cast iron, various metal alloys and composite materials. In particular, brake rotor designers look for toughness, low density (low weight), low wear, and high coefficient of thermal conductivity in the material they select for the brake rotor. Generally, brake rotors are cast in iron. However, new materials have been developed that allow casting of rotors from an aluminum metal matrix composite (MMC) material such as 359 aluminum with twenty percent silicon carbide particulate reinforcement. See, for example, U.S. Pat. No. 5,407,035 and U.S. Pat. No. 5,526,914.
The problem with focusing on the criteria of toughness, low density, low wear, and high thermal conductivity for brake rotor material is that these criteria do not account for the radially compressive impact forces experienced by the slots in the brake rotor. Each time the brake pads are applied to the brake rotor, the drive pins are pressed into the circumferential width of the slots. The compressive force of each drive pin acting on its associated slot width works to circumferentially distort the soft, malleable aluminum MMC material forming each slot in high speed/temperature applications.
Since brakes primarily are applied as an automobile is traveling in one direction, the deformation of each slot width in one radial direction usually predominates over the deformation of each slot width in the opposite radial direction. As the width of each slot increases due to compression slot expansion, the brake rotor freely begins to move rotationally relative to the bolts and the brake pads. Now, the width of each radially slot does not work to control the rotational movement of the brake rotor relative to the bolts and the brake pads.
Without the rotor being rotationally fixed relative to the brake pads, the braking cycle is adversely affected. The rotational movement of the rotor causes judder and vibration, each of which works to lessen the radially slot control over the axial movement of the brake rotor relative to the brake pads. The lessening of the radially slot control over the axial movement of the brake rotor relative to the brake pads causes imbalance in the rotor. As the slots widen over time, the time it takes to brake over a given length and speed increases. Noise and judder eventually become so pronounced that the brake system becomes inoperable. Eventually, brake rotors degraded by compression slot expansion have to be replaced. Thus, there is a need to minimize or eliminate compression slot expansion.
The present invention relates to a rotor assembly. The rotor assembly includes a rotor having rotor holes disposed about an axis. Bushings are disposed within the rotor holes in order to protect the rotor holes from compression impact deformation. Each bushing includes an internal slot that permits the rotor to radially expand and contract due to changes in the temperature of the rotor. A hub having pin holes that align with the rotor holes is held to the rotor by drive pins disposed within the pin holes and the bushings. A bolt is placed into the drive pin and tightened to fix the hub to the rotor in the axial and circumferential directions. Other features are disclosed.